Stilotherium parvum, Abello & Martin & Cardoso, 2021

Abello, María Alejandra, Martin, Gabriel M. & Cardoso, Yamila, 2021, Review of the extinct ‘ shrew-opossums’ (Marsupialia: Caenolestidae), with descriptions of two new genera and three new species from the Early Miocene of southern South America, Zoological Journal of the Linnean Society 193 : -

STILOTHERIUM PARVUM SP. NOV.

( FIG. 7A–O)

Zoobank registration: lsid:urn:lsid:zoobank. o r g: a c t: 4 D 6 D 1 4 F E - A F D F - 4 C 1 D - 9 9 F 1 - 3212EB3CC7FC

Etymology: From Latin parvus, small, in reference to its smaller size compared to S. dissimile .

Holotype: MPEF-PV 5830 , right mandible with alveoli for i2-3, two a.u., anterior root and posterior alveolus for p2, roots of p3, complete m1-3 and alveolus for m4 ( Fig. 7A–C).

Referred material: MLP 77-VI-13-10, right mandible with m3-4, and alveolus for m2 ( Fig. 7D–F); MPEF-PV 4756, left mandible with m2-4 ( Fig. 7G, H); MPEF-PV 4796, isolated right m2; MPEF-PV 4802, isolated left M1; MPEF-PV 4818, right mandible with alveoli for m2 and m4, and talonid of m3; MPEF-PV 4833, right mandible with alveoli for m3 and complete m4; MPEF-PV 4861, isolated left M1 ( Fig. 7I–K, O); MPEF-PV 4888, isolated right M2; MPEF-PV 4801, isolated right M2 ( Fig. 7L–N), and MEF 4701, isolated right M?2.

Temporal distribution: Early Miocene, Colhuehuapian SALMA.

Stratigraphic and geographic distribution: Sarmiento Formation, Colhue-Huapi Member. Gran Barranca, Chubut province, Argentina.

Measurements: Supporting Information, File S6, Tables 1a, 2.

Diagnosis: Stilotherium parvum differs from S. dissimile by (1) its smaller size, (2) larger retromolar foramen, (3) p2 shorter than p3, (4) presence of postcingulum on m2-3, (5) paraconid slightly developed in m4, (6) larger metaconule on M1-2 and (7) presence of paracone on M2.

Description: The mandibular body of the type specimen is deep and of relatively uniform height throughout its length. Labially, it shows two mental foramina, a moderately sized foramen below the anterior root of m1 and a second smaller foramen below the posterior root of the same molar. The mandibular symphysis extends posteriorly to the level of the anterior root of p3. Behind the alveolus for i2 there are two small alveoli for uniradiculate teeth, located one next to the other and transversely aligned ( Fig. 7A). The lingual alveolus has the staggered condition, thus corresponding to i3; the labial one, on the other hand, probably corresponds to i4. Between the alveoli for i3-4 and the preserved roots of p3, there are four alveoli which might represent the canine and p1, both uniradiculate, and the biradiculate p2. Judging from their alveolar size, p2 should have been shorter than p3.

The molars are worn, making descriptions of cusps and crests somewhat difficult. The anterobasal cingulum of m1 is short, extending posteriorly to the protoconid base, and the anterobasal cinguli of m2-3 are long, extending posteriorly to the hypoconid. Trigonids are shorter and narrower than talonids in m1-3, showing a progression in the molar series, from short and narrow trigonid in m1, to wider trigonid in m3. The trigonid of m1 is different in its cusp and cristid morphology from that of m2-3. In m1, the paraconid is reduced, poorly differentiated with respect to the paracristid and has an almost straight paracristid with no distinct postparacristid and preprotocristid. In m2-3, para- and metaconid are subequal in size, both well differentiated from their corresponding cristids; the paracristid is divided into a postparacristid, oriented transversely to the main molar axis, and a preprotocristid relatively parallel to this axis (see Figs 3E, 7A). In spite of their toothwear, the entoconids are preserved in all molars, and their entocristids are in contact with the posterior wall of the metaconids. In m3, which is the less worn molar, a lingually curved entocristid is clearly identifiable ( Fig. 7A). In m1-3 the hypoconids are large and salient, the hypoconulids are poorly developed and anteroposteriorly compressed. A delicate postcingulum occurs in the posterior wall of m3, below the postcristid ( Fig. 7A). Although m4 was not preserved, based on the size of its alveolus (half the size of the posterior alveolus of m3) it should have been small, as observed in the referred materials (MPEF-PV 4756, MPEF-PV 4833 and MLP 77-VI-13-10). Behind the alveolus of m4, a well-developed foramen was preserved in the retromolar area (retromolar foramen, see: Beck, 2017).

The specimen MPEF-PV 4756 has a deep mandible while MLP 77 -VI-13-10 has relatively shallow mandibular ramus (see Supporting Information, File S 6, Table 1a). Both specimens coincide with the type in all comparable molar traits: trigonids labiolingually narrower than talonids (particularly in m2), long anterobasal cinguli on m2-3, para- and metaconid subequal in size and transversely aligned, slender entoconids and lingually curved entocristids. Because molars are less worn than those of the type, specially the m3 preserved in MLP 77 -VI-13-10, we were able to observe that the distal end of the entocristid and cristid obliqua are of equal height, and that the postcristid, particularly in m3, is slightly olique (i.e. anterolabially to posterodistally oriented). As in the type specimen, a postcingulum is present in m3, but in contrast to the type specimen, a postcingulum is clearly visible in m2 of MPEF-PV 4756 ( Fig. 7G, H). The preserved m4 is uniradiculate, small-sized to vestigial ( Fig. 7F–H). The lingual face of MPEF-PV 4756 is worn, but a small paraconid and a large protoconid can still be differentiated; the talonid in this specimen is reduced to a posterior shelf. In MLP 77 -VI-13- 10 m 4 is even more reduced ( Fig. 7E, F). As well as in S. dissimile , we interpret that some morphological differences among referred specimens are due to their intraspecific variability. Among these differing characters are the mandibular height, deeper in the type specimen and MPEF-PV 4756 , but shallow in MLP 77 -VI-13-10 and MPEF-PV 4818 ; larger molars in the type and MLP 77 -VI-13-10 than those of MPEF-PV 4756 , and differences in the development of m4 (see above and Supporting Information, File S 6, Table 1a) .

Specimens MPEF-PV 4802, MPEF-PV 4861, MPEF-PV 4888 and MPEF-PV 4801 are isolated upper molars. Based on the comparison with the dental series of S. dissimile (contact relationships between teeth and relative size of metaconule), the first two are inferred to be M1 and the last two M2. The M1 are characterized by having quadrangular shape in occlusal view, due to the lingually salient metaconule ( Fig. 3A). The metaconule is a shallow cusp, much lower than the protocone, and extended posteriorly beyond the posterior wall of the metacone and StC+D ( Fig. 7I–J). The metaconule has a cingulumlike appearance on the posterior corner of the molar, clearly identifiable in unworn teeth, and has a flexus in the postmetaconular crest ( Fig. 7I–L). The stylar cusps are labiolingually compressed, and the StC+D is stouter and higher than StB ( Fig. 3B). In labial view, the molar shows two cinguli, one developed from the distal end of the anterior crest of StB (anterolingual cingulum; see Fig. 7O) and the other from the distal end of the posterior crest of StC+D (posterolingual cingulum; see Fig. 7Q), both ending at the base of the crown. The metacone is a well-developed cusp, but lower and less stout than StC+D, partially coalescent and paired with StC+D. The postmetacrista is relatively long and ends at the metastylar corner of the molar ( Fig. 3A). The premetacrista is shorter than the postmetacrista, ending at the anterolingual face of StC+D. The postmetaconular crest extends below the postmetcrista and along the posterior wall of the metacone. The contact area with M2 (posteriorly) is delimited by the distal end of the postmetaconular crest and the posterolingual cingulum ( Fig. 7K). The contact area with P3 (anteriorly) is at a lower level than that with M2 ( Fig 7O), has the shape of a vertical groove and is lingually delimited by the anterolingual cingulum. The M2 resembles M 1 in having a flexus in the postmetaconular crest, transversely aligned StC+D and metacone, and in the metacone relative size; it can be distinguished from M1 by having a less salient metaconule, StB and StC+D of subequal size ( Fig. 3A, B). The paracone is vestigial and preserved as a crest on the labial wall of StB ( Fig. 7L–N). It is closely appressed to StB, positioned on the anterolingual face of this stylar cup, and its postparacrista can be distinguished as a delicate crest that runs obliquely over StB ( Fig. 7M). The contact area with the adjacent molars is groove-like and located nearly at the same level ( Fig. 7B). The contact area between M1 and M2 is lingually delimited by the anterolingual cingulum, and the contact area with M3 is delimited by the distal end of the postmetaconular crest and the posterolingual cingulum ( Fig. 7N). MPEF 4701 is an upper molar of uncertain locus (the contact areas with the adjacent molars are poorly preserved), although probably assignable to M2 by the size of its metaconule.

Remarks: Ameghino(1902b:123) mentionedthepresence of Garzonia sp. (a junior synonym of Stilotherium , see above) in Colhuehuapian levels of Patagonia. In his original description, he mentioned its small size, an indication that the material could be referred to a specimen of S. parvum . However, no specimen labelled Garzonia sp. with the described stratigraphic procedence has been found in the MACN collection to date. The new species described herein is 20% smaller, in linear dimensions than S. dissimile (see Supporting Information, File S6, Table 1a). One of the most notable differences with the type species ( S. dissimile ) is the presence of a postcingulum on m2-3, a unique character among paucituberculatans. Other differences include a more posteriorly expanded metaconule on M1-2 of S. parvum . In S. parvum , metaconules extend to the level of the metastylar region and have a smaller occlusal surface than those of S. dissimile .